Modern trim panels are generally laminar composites which can be contoured for attachment to the interior metal framing of an automotive vehicle, particularly by "snap-in" techniques, as by temporarily flexing a panel for insertion of peripheral edges into trim channels and then permitting the panel to straighten. The panel thus becomes self supporting. Typically, such panels are used as headliners to cover the underside of metal roof panels but they are also used as coverings for wall panels, sometimes affixed with screws, clips or other mechanical fasteners.
As described in the technical and patent literature, such as U.S. Pat. Nos. 4,172,918 to Doerer and 4,256,797 to Stamper et al., trim panels combine decorative appearance with sound insulation, temperature control, concealment of imperfections, heat resistance to avoid sag and blistering, and sufficient flexibility, resilience and strength to withstand handling during thermoforming and installation and to be self-supporting (non-fluttering) after installation. Generally, no single material will provide all of the required qualities together with acceptable cost and therefore trim panels normally are laminar composites which combine the properties of the individual layers such that the deficiencies of any one layer are offset by the benefits of one or more other layers. For example, a layer which provides good sound absorption often will not have sufficient strength and resistance to bending to enable it to be installed by the "snap-in" technique.
Accordingly, the aforementioned patents describe laminar composites based upon a relatively semi-flexible or stiff substrate sheet or core to which are bonded on one or both sides a foam layer and a decorative layer to provide the desired sound absorbing and aesthetic qualities. Nevertheless, it has been found that although the foregoing composites are effective for absorbing airborne noise originating from the engine, wind, weather conditions, the road or other exterior or interior sources, they are not highly effective in reducing noise which is developed within the headliners themselves by reason of acoustical excitation or resonance induced by structural vibration of an automotive frame, such as occurs when rain, wind, engine vibrations and/or rough road conditions cause an automobile roof or frame, covered by a trim panel, to vibrate. In fact, studies have shown that the better the quality of the support layer used in the trim panel, the greater the tendency of the trim panel to vibrate. For example, the higher strength, stiffer support material used in the trim panels of the foregoing patents is more easily excited and radiates noise more efficiently than a glass fiber support material. However, the weaker glass fiber material is subject to sag unless supported by another material.
As will become apparent from the discussion below, the problem of reducing noise generated by a trim panel via automobile structural vibration is not resolved merely by introducing any vibration damping material between the trim panel and the metal frame of the automobile which is subject to vibration. The damping material must have certain characteristics which cooperate with the properties of other layers and materials used in the trim panel construction for proper balance of the totality of qualities desired.